Accessing a migrated member in an updated type

ABSTRACT

Accessing migrated members in an updated type is described. Instructions to access a migrated member may be: (a) storing a value of a particular type as a value of a migrated field, or (b) invoking a migrated method using an argument of a particular type. The argument of the particular type, specified in the instructions, is converted into a value of the type associated with the current version of the migrated member. The migrated member is accessed using the converted value. Alternatively, instructions may be: (a) fetching and returning a value of a migrated field as a value of a particular type, or (b) returning a value from a migrated method as a value of a particular type. A value is returned via accessing the current version of the migrated member. The returned value is converted into a value of the particular type specified in the instructions.

INCORPORATION BY REFERENCE; DISCLAIMER

Each of the following applications are hereby incorporated by reference:application Ser. No. 15/426,312 filed on Feb. 7, 2017; application No.62/361,087 filed Jul. 12, 2016; application Ser. No. 15/426,839 filed onFeb. 7, 2017. The Applicant hereby rescinds any disclaimer of claimscope in the parent application(s) or the prosecution history thereofand advises the USPTO that the claims in this application may be broaderthan any claim in the parent application(s).

TECHNICAL FIELD

The present disclosure relates to compiling and/or executing a type. Inparticular, the present disclosure relates to accessing a migratedmember of an updated type.

BACKGROUND

A compiler converts source code, which is written according to aspecification directed to the convenience of the programmer, to eithermachine or object code, which is executable directly by the particularmachine environment, or an intermediate representation (“virtual machinecode/instructions”), such as bytecode, which is executable by a virtualmachine that is capable of running on top of a variety of particularmachine environments. The virtual machine instructions are executable bythe virtual machine in a more direct and efficient manner than thesource code. Converting source code to virtual machine instructionsincludes mapping source code functionality from the language to virtualmachine functionality that utilizes underlying resources, such as datastructures. Often, functionality that is presented in simple terms viasource code by the programmer is converted into more complex steps thatmap more directly to the instruction set supported by the underlyinghardware on which the virtual machine resides.

The approaches described in this section are approaches that could bepursued, but not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings. It should benoted that references to “an” or “one” embodiment in this disclosure arenot necessarily to the same embodiment, and they mean at least one. Inthe drawings:

FIG. 1 illustrates an example computing architecture in which techniquesdescribed herein may be practiced.

FIG. 2 is a block diagram illustrating one embodiment of a computersystem suitable for implementing methods and features described herein.

FIG. 3 illustrates an example virtual machine memory layout in blockdiagram form according to an embodiment.

FIG. 4 illustrates an example frame in block diagram form according toan embodiment.

FIG. 5 illustrates an example of an updated type, in accordance with oneor more embodiments;

FIG. 6 illustrates a set of operations for compiling an updated typecomprising an migrated member, in accordance with one or moreembodiments.

FIGS. 7A-B illustrate a set of operations for fetching and returning avalue of a migrated field as a value of a particular type, in accordancewith one or more embodiments.

FIGS. 8A-B illustrate a set of operations for storing a value of aparticular type as a value of a migrated field, in accordance with oneor more embodiments.

FIGS. 9A-B illustrate a set of operations for invoking a method usingvalues of a particular set of types as arguments to the method, inaccordance with one or more embodiments.

FIGS. 10A-B illustrate a set of operations for returning a value from amethod as a value of a particular type, in accordance with one or moreembodiments.

FIG. 11 illustrates a system in accordance with one or more embodiments.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding. One or more embodiments may be practiced without thesespecific details. Features described in one embodiment may be combinedwith features described in a different embodiment. In some examples,well-known structures and devices are described with reference to ablock diagram form in order to avoid unnecessarily obscuring the presentinvention.

-   -   1. GENERAL OVERVIEW    -   2. ARCHITECTURAL OVERVIEW        -   2.1 EXAMPLE CLASS FILE STRUCTURE        -   2.2 EXAMPLE VIRTUAL MACHINE ARCHITECTURE        -   2.3 LOADING, LINKING, AND INITIALIZING    -   3. AN UPDATED TYPE    -   4. COMPILING AN UPDATED TYPE    -   5. ACCESSING A MIGRATED FIELD    -   6. ACCESSING A MIGRATED METHOD    -   7. EXAMPLE EMBODIMENTS    -   8. MISCELLANEOUS; EXTENSIONS

9. HARDWARE OVERVIEW 1. General Overview

One or more embodiments include accessing a migrated member in anupdated type. A migrated member (such as a migrated field or a migratedmethod) is a member for which multiple versions exist. Multiple versionsof the member within the same updated type may include, for example, (a)a current version and (b) an outdated version which has been migrated tothe current version. Each version of the migrated member is associatedwith a same member name. An updated type, as referred to herein forpurposes of explanation, is a type that includes multiple versions of amigrated member.

In an embodiment, accessing a migrated field includes fetching andreturning a value of a migrated field, corresponding to an object, as avalue of a particular type. The field type associated with a currentversion of the migrated field is determined to be different from theparticular type. Responsive to determining that the field typeassociated with the current version of the migrated field is differentfrom the particular type, the value of the migrated field is convertedinto a value of the particular type. The converted value is stored asthe value of the particular type.

In an embodiment, accessing a migrated field includes storing a value ofa particular type as a value of a migrated field corresponding to anobject. The field type associated with a current version of the migratedfield is determined to be different from the particular type. Responsiveto determining that the field type associated with the current versionof the migrated field is different from the particular type, the valueof the particular type is converted into a value of the field typeassociated with the current version of the migrated field. The convertedvalue is stored as the value of the current version of the migratedfield.

In an embodiment, accessing a migrated method includes invoking amigrated method using values of a particular set of types as argumentsfor the method. A set of parameter types associated with a currentversion of the migrated method is determined to be different from theparticular set of types. Responsive to determining that the set ofparameter types associated with the current version of the migratedmethod is different from the particular set of types, the values of theparticular set of types are converted into values of the set ofparameter types. The converted values are used as arguments for invokingthe current version of the migrated method.

In an embodiment, accessing a migrated method includes returning a valuefrom a migrated method as a value of a particular type. A return typeassociated with a current version of the migrated method is determinedto be different from the particular type. Responsive to determining thatthe return type associated with the current version of the migratedmethod is different from the particular type, the value returned by thecurrent version of the migrated method is converted into a value of theparticular type. The converted value is stored as the value of theparticular type.

One or more embodiments described in this Specification and/or recitedin the claims may not be included in this General Overview section.

2. Architectural Overview

FIG. 1 illustrates an example architecture in which techniques describedherein may be practiced. Software and/or hardware components describedwith relation to the example architecture may be omitted or associatedwith a different set of functionality than described herein. Softwareand/or hardware components, not described herein, may be used within anenvironment in accordance with one or more embodiments. Accordingly, theexample environment should not be constructed as limiting the scope ofany of the claims.

As illustrated in FIG. 1, a computing architecture 100 includes sourcecode files 101 which are compiled by a compiler 102 into class files 103representing the program to be executed. The class files 103 are thenloaded and executed by an execution platform 112, which includes aruntime environment 113, an operating system 111, and one or moreapplication programming interfaces (APIs) 110 that enable communicationbetween the runtime environment 113 and the operating system 111. Theruntime environment 113 includes a virtual machine 104 comprisingvarious components, such as a memory manager 105 (which may include agarbage collector), a class file verifier 106 to check the validity ofclass files 103, a class loader 107 to locate and build in-memoryrepresentations of classes, an interpreter 108 for executing the virtualmachine 104 code, and a just-in-time (JIT) compiler 109 for producingoptimized machine-level code.

In an embodiment, the computing architecture 100 includes source codefiles 101 that contain code that has been written in a particularprogramming language, such as Java, C, C++, C#, Ruby, Perl, and soforth. Thus, the source code files 101 adhere to a particular set ofsyntactic and/or semantic rules for the associated language. Forexample, code written in Java adheres to the Java LanguageSpecification. However, since specifications are updated and revisedover time, the source code files 101 may be associated with a versionnumber indicating the revision of the specification to which the sourcecode files 101 adhere. The exact programming language used to write thesource code files 101 is generally not critical.

In various embodiments, the compiler 102 converts the source code, whichis written according to a specification directed to the convenience ofthe programmer, to either machine or object code, which is executabledirectly by the particular machine environment, or an intermediaterepresentation (“virtual machine code/instructions”), such as bytecode,which is executable by a virtual machine 104 that is capable of runningon top of a variety of particular machine environments. The virtualmachine instructions are executable by the virtual machine 104 in a moredirect and efficient manner than the source code. Converting source codeto virtual machine instructions includes mapping source codefunctionality from the language to virtual machine functionality thatutilizes underlying resources, such as data structures. Often,functionality that is presented in simple terms via source code by theprogrammer is converted into more complex steps that map more directlyto the instruction set supported by the underlying hardware on which thevirtual machine 104 resides.

In general, programs are executed either as a compiled or an interpretedprogram. When a program is compiled, the code is transformed globallyfrom a first language to a second language before execution. Since thework of transforming the code is performed ahead of time; compiled codetends to have excellent run-time performance. In addition, since thetransformation occurs globally before execution, the code can beanalyzed and optimized using techniques such as constant folding, deadcode elimination, inlining, and so forth. However, depending on theprogram being executed, the startup time can be significant. Inaddition, inserting new code would require the program to be takenoffline, re-compiled, and re-executed. For many dynamic languages (suchas Java) which are designed to allow code to be inserted during theprogram's execution, a purely compiled approach may be inappropriate.When a program is interpreted, the code of the program is readline-by-line and converted to machine-level instructions while theprogram is executing. As a result, the program has a short startup time(can begin executing almost immediately), but the run-time performanceis diminished by performing the transformation on the fly. Furthermore,since each instruction is analyzed individually, many optimizations thatrely on a more global analysis of the program cannot be performed.

In some embodiments, the virtual machine 104 includes an interpreter 108and a JIT compiler 109 (or a component implementing aspects of both),and executes programs using a combination of interpreted and compiledtechniques. For example, the virtual machine 104 may initially begin byinterpreting the virtual machine instructions representing the programvia the interpreter 108 while tracking statistics related to programbehavior, such as how often different sections or blocks of code areexecuted by the virtual machine 104. Once a block of code surpasses athreshold (is “hot”), the virtual machine 104 invokes the JIT compiler109 to perform an analysis of the block and generate optimizedmachine-level instructions which replaces the “hot” block of code forfuture executions. Since programs tend to spend most time executing asmall portion of overall code, compiling just the “hot” portions of theprogram can provide similar performance to fully compiled code, butwithout the start-up penalty. Furthermore, although the optimizationanalysis is constrained to the “hot” block being replaced, there stillexists far greater optimization potential than converting eachinstruction individually. There are a number of variations on the abovedescribed example, such as tiered compiling.

In order to provide clear examples, the source code files 101 have beenillustrated as the “top level” representation of the program to beexecuted by the execution platform 112. Although the computingarchitecture 100 depicts the source code files 101 as a “top level”program representation, in other embodiments the source code files 101may be an intermediate representation received via a “higher level”compiler that processed code files in a different language into thelanguage of the source code files 101. Some examples in the followingdisclosure assume that the source code files 101 adhere to a class-basedobject-oriented programming language. However, this is not a requirementto utilizing the features described herein.

In an embodiment, compiler 102 receives as input the source code files101 and converts the source code files 101 into class files 103 that arein a format expected by the virtual machine 104. For example, in thecontext of the JVM, the Java Virtual Machine Specification defines aparticular class file format to which the class files 103 are expectedto adhere. In some embodiments, the class files 103 contain the virtualmachine instructions that have been converted from the source code files101. However, in other embodiments, the class files 103 may containother structures as well, such as tables identifying constant valuesand/or metadata related to various structures (classes, fields, methods,and so forth).

The following discussion assumes that each of the class files 103represents a respective “type” defined in the source code files 101 (ordynamically generated by the compiler 102/virtual machine 104). Examplesof “types” include a class and an interface. A class is a template forthe properties and behaviors of objects associated with the class. Theclass includes fields and methods associated with the objects of theclass. An interface includes abstract methods that may be implemented bya class. A class that implements an interface inherits the abstractmethods of the interface and provides a body to each abstract method.However, the aforementioned assumption is not a strict requirement andwill depend on the implementation of the virtual machine 104. Thus, thetechniques described herein may still be performed regardless of theexact format of the class files 103.

Types may be updated to include multiple fields with a same field namebut different field types. Additionally or alternatively, types may beupdated to include multiple methods with a same method name butdifferent parameter types and/or return types. Embodiments herein relateto accessing a member of an updated type.

In some embodiments, the class files 103 are divided into one or more“libraries” or “packages”, each of which includes a collection ofclasses that provide related functionality. For example, a library maycontain one or more class files that implement input/output (I/O)operations, mathematics tools, cryptographic techniques, graphicsutilities, and so forth. Further, some classes (or fields/methods withinthose classes) may include access restrictions that limit their use towithin a particular class/library/package or to classes with appropriatepermissions.

2.1 Example Class File Structure

FIG. 2 illustrates an example structure for a class file 200 in blockdiagram form according to an embodiment. In order to provide clearexamples, the remainder of the disclosure assumes that the class files103 of the computing architecture 100 adhere to the structure of theexample class file 200 described in this section. However, in apractical environment, the structure of the class file 200 will bedependent on the implementation of the virtual machine 104. Further, oneor more features discussed herein may modify the structure of the classfile 200 to, for example, add additional structure types. Therefore, theexact structure of the class file 200 is not critical to the techniquesdescribed herein. For the purposes of Section 2.1, “the class” or “thepresent class” refers to the class represented by the class file 200.

In FIG. 2, the class file 200 includes a constant table 201, fieldstructures 208, class metadata 207, and method structures 209. In anembodiment, the constant table 201 is a data structure which, amongother functions, acts as a symbol table for the class. For example, theconstant table 201 may store data related to the various identifiersused in the source code files 101 such as type, scope, contents, and/orlocation. The constant table 201 has entries for value structures 202(representing constant values of type int, long, double, float, byte,string, and so forth), class information structures 203, name and typeinformation structures 204, field reference structures 205, and methodreference structures 206 derived from the source code files 101 by thecompiler 102. In an embodiment, the constant table 201 is implemented asan array that maps an index i to structure j. However, the exactimplementation of the constant table 201 is not critical.

In some embodiments, the entries of the constant table 201 includestructures which index other constant table 201 entries. For example, anentry for one of the value structures 202 representing a string may holda tag identifying its “type” as string and an index to one or more othervalue structures 202 of the constant table 201 storing char, byte or intvalues representing the ASCII characters of the string.

In an embodiment, field reference structures 205 of the constant table201 hold an index into the constant table 201 to one of the classinformation structures 203 representing the class defining the field andan index into the constant table 201 to one of the name and typeinformation structures 204 that provides the name and descriptor of thefield. Method reference structures 206 of the constant table 201 hold anindex into the constant table 201 to one of the class informationstructures 203 representing the class defining the method and an indexinto the constant table 201 to one of the name and type informationstructures 204 that provides the name and descriptor for the method. Theclass information structures 203 hold an index into the constant table201 to one of the value structures 202 holding the name of theassociated class.

The name and type information structures 204 hold an index into theconstant table 201 to one of the value structures 202 storing the nameof the field/method and an index into the constant table 201 to one ofthe value structures 202 storing the descriptor.

In an embodiment, class metadata 207 includes metadata for the class,such as version number(s), number of entries in the constant pool,number of fields, number of methods, access flags (whether the class ispublic, private, final, abstract, etc.), an index to one of the classinformation structures 203 of the constant table 201 that identifies thepresent class, an index to one of the class information structures 203of the constant table 201 that identifies the superclass (if any), andso forth.

In an embodiment, the field structures 208 represent a set of structuresthat identifies the various fields of the class. The field structures208 store, for each field of the class, accessor flags for the field(whether the field is static, public, private, final, etc.), an indexinto the constant table 201 to one of the value structures 202 thatholds the name of the field, and an index into the constant table 201 toone of the value structures 202 that holds a descriptor of the field.

In an embodiment, the method structures 209 represent a set ofstructures that identifies the various methods of the class. The methodstructures 209 store, for each method of the class, accessor flags forthe method (e.g. whether the method is static, public, private,synchronized, etc.), an index into the constant table 201 to one of thevalue structures 202 that holds the name of the method, an index intothe constant table 201 to one of the value structures 202 that holds thedescriptor of the method, and the virtual machine instructions thatcorrespond to the body of the method as defined in the source code files101.

In an embodiment, a descriptor represents a type of a field or method.For example, the descriptor may be implemented as a string adhering to aparticular syntax. While the exact syntax is not critical, a fewexamples are described below.

In an example where the descriptor represents a type of the field, thedescriptor identifies the type of data held by the field. In anembodiment, a field can hold a basic type, an object, or an array. Whena field holds a basic type, the descriptor is a string that identifiesthe basic type (e.g., “B”=byte, “C”=char, “D”=double, “F”=float,“I”=int, “J”=long int, etc.). When a field holds an object, thedescriptor is a string that identifies the class name of the object(e.g. “L ClassName”). “L” in this case indicates a reference, thus “LClassName” represents a reference to an object of class ClassName. Whenthe field is an array, the descriptor identifies the type held by thearray. For example, “[B” indicates an array of bytes, with “[”indicating an array and “B” indicating that the array holds the basictype of byte. However, since arrays can be nested, the descriptor for anarray may also indicate the nesting. For example, “[[L ClassName”indicates an array where each index holds an array that holds objects ofclass ClassName. In some embodiments, the ClassName is fully qualifiedand includes the simple name of the class, as well as the pathname ofthe class. For example, the ClassName may indicate where the file isstored in the package, library, or file system hosting the class file200.

In the case of a method, the descriptor identifies the parameters of themethod and the return type of the method. For example, a methoddescriptor may follow the general form “({ParameterDescriptor})ReturnDescriptor”, where the {ParameterDescriptor} is a set or list offield descriptors representing the parameters and the ReturnDescriptoris a field descriptor identifying the return type. For instance, thestring “V” may be used to represent the void return type. Thus, a methoddefined in the source code files 101 as “Object m(int I, double d,Thread t) { . . . }” matches the descriptor “(I D L Thread) L Object”.

In an embodiment, the virtual machine instructions held in the methodstructures 209 include operations which reference entries of theconstant table 201. Using Java as an example, consider the followingclass:

class A

-   -   {int add12and13( ) {return B.addTwo(12, 13); }}

In the above example, the Java method add12and13 is defined in class A,takes no parameters, and returns an integer. The body of methodadd12and13 calls static method addTwo of class B which takes theconstant integer values 12 and 13 as parameters, and returns the result.Thus, in the constant table 201, the compiler 102 includes, among otherentries, a method reference structure that corresponds to the call tothe method B.addTwo. In Java, a call to a method compiles down to aninvoke command in the bytecode of the JVM (in this case invokestatic asaddTwo is a static method of class B). The invoke command is provided anindex into the constant table 201 corresponding to the method referencestructure that identifies the class defining addTwo “B”, the name ofaddTwo “addTwo”, and the descriptor of addTwo “(I I)I”. For example,assuming the aforementioned method reference is stored at index 4, thebytecode instruction may appear as “invokestatic #4”.

Since the constant table 201 refers to classes, methods, and fieldssymbolically with structures carrying identifying information, ratherthan direct references to a memory location, the entries of the constanttable 201 are referred to as “symbolic references”. One reason thatsymbolic references are utilized for the class files 103 is because, insome embodiments, the compiler 102 is unaware of how and where theclasses will be stored once loaded into the runtime environment 113. Aswill be described in Section 2.3, eventually the run-timerepresentations of the symbolic references are resolved into actualmemory addresses by the virtual machine 104 after the referenced classes(and associated structures) have been loaded into the runtimeenvironment and allocated concrete memory locations.

2.2 Example Virtual Machine Architecture

FIG. 3 illustrates an example virtual machine memory layout 300 in blockdiagram form according to an embodiment. In order to provide clearexamples, the remaining discussion will assume that the virtual machine104 adheres to the virtual machine memory layout 300 depicted in FIG. 3.In addition, although components of the virtual machine memory layout300 may be referred to as memory “areas”, there is no requirement thatthe memory areas are contiguous.

In the example illustrated by FIG. 3, the virtual machine memory layout300 is divided into a shared area 301 and a thread area 307. The sharedarea 301 represents an area in memory where structures shared among thevarious threads executing on the virtual machine 104 are stored. Theshared area 301 includes a heap 302 and a per-class area 303. In anembodiment, the heap 302 represents the run-time data area from whichmemory for class instances and arrays is allocated. In an embodiment,the per-class area 303 represents the memory area where the datapertaining to the individual classes are stored. In an embodiment, theper-class area 303 includes, for each loaded class, a run-time constantpool 304 representing data from the constant table 201 of the class,field and method data 306 (for example, to hold the static fields of theclass), and the method code 305 representing the virtual machineinstructions for methods of the class.

The thread area 307 represents a memory area where structures specificto individual threads are stored. In FIG. 3, the thread area 307includes thread structures 308 and thread structures 311, representingthe per-thread structures utilized by different threads. In order toprovide clear examples, the thread area 307 depicted in FIG. 3 assumestwo threads are executing on the virtual machine 104. However, in apractical environment, the virtual machine 104 may execute any arbitrarynumber of threads, with the number of thread structures scaledaccordingly.

In an embodiment, thread structures 308 includes program counter 309 andvirtual machine stack 310. Similarly, thread structures 311 includesprogram counter 312 and virtual machine stack 313. In an embodiment,program counter 309 and program counter 312 store the current address ofthe virtual machine instruction being executed by their respectivethreads.

Thus, as a thread steps through the instructions, the program countersare updated to maintain an index to the current instruction. In anembodiment, virtual machine stack 310 and virtual machine stack 313 eachstore frames for their respective threads that hold local variables andpartial results, and is also used for method invocation and return.

In an embodiment, a frame is a data structure used to store data andpartial results, return values for methods, and perform dynamic linking.A new frame is created each time a method is invoked. A frame isdestroyed when the method that caused the frame to be generatedcompletes. Thus, when a thread performs a method invocation, the virtualmachine 104 generates a new frame and pushes that frame onto the virtualmachine stack associated with the thread.

When the method invocation completes, the virtual machine 104 passesback the result of the method invocation to the previous frame and popsthe current frame off of the stack. In an embodiment, for a giventhread, one frame is active at any point. This active frame is referredto as the current frame, the method that caused generation of thecurrent frame is referred to as the current method, and the class towhich the current method belongs is referred to as the current class.

FIG. 4 illustrates an example frame 400 in block diagram form accordingto an embodiment. In order to provide clear examples, the remainingdiscussion will assume that frames of virtual machine stack 310 andvirtual machine stack 313 adhere to the structure of frame 400.

In an embodiment, frame 400 includes local variables 401, operand stack402, and run-time constant pool reference table 403. In an embodiment,the local variables 401 are represented as an array of variables thateach hold a value, for example, Boolean, byte, char, short, int, float,or reference. Further, some value types, such as longs or doubles, maybe represented by more than one entry in the array. The local variables401 are used to pass parameters on method invocations and store partialresults. For example, when generating the frame 400 in response toinvoking a method, the parameters may be stored in predefined positionswithin the local variables 401, such as indexes 1-N corresponding to thefirst to Nth parameters in the invocation.

In an embodiment, the operand stack 402 is empty by default when theframe 400 is created by the virtual machine 104. The virtual machine 104then supplies instructions from the method code 305 of the currentmethod to load constants or values from the local variables 401 onto theoperand stack 402. Other instructions take operands from the operandstack 402, operate on them, and push the result back onto the operandstack 402. Furthermore, the operand stack 402 is used to prepareparameters to be passed to methods and to receive method results. Forexample, the parameters of the method being invoked could be pushed ontothe operand stack 402 prior to issuing the invocation to the method. Thevirtual machine 104 then generates a new frame for the method invocationwhere the operands on the operand stack 402 of the previous frame arepopped and loaded into the local variables 401 of the new frame. Whenthe invoked method terminates, the new frame is popped from the virtualmachine stack and the return value is pushed onto the operand stack 402of the previous frame.

In an embodiment, the run-time constant pool reference table 403contains a reference to the run-time constant pool 304 of the currentclass. The run-time constant pool reference table 403 is used to supportresolution. Resolution is the process whereby symbolic references in theconstant pool 304 are translated into concrete memory addresses, loadingclasses as necessary to resolve as-yet-undefined symbols and translatingvariable accesses into appropriate offsets into storage structuresassociated with the run-time location of these variables.

2.3 Loading, Linking, and Initializing

In an embodiment, the virtual machine 104 dynamically loads, links, andinitializes classes. Loading is the process of finding a class with aparticular name and creating a representation from the associated classfile 200 of that class within the memory of the runtime environment 113.For example, creating the run-time constant pool 304, method code 305,and field and method data 306 for the class within the per-class area303 of the virtual machine memory layout 300. Linking is the process oftaking the in-memory representation of the class and combining it withthe run-time state of the virtual machine 104 so that the methods of theclass can be executed. Initialization is the process of executing theclass constructors to set the starting state of the field and methoddata 306 of the class and/or create class instances on the heap 302 forthe initialized class.

The following are examples of loading, linking, and initializingtechniques that may be implemented by the virtual machine 104. However,in many embodiments the steps may be interleaved, such that an initialclass is loaded, then during linking a second class is loaded to resolvea symbolic reference found in the first class, which in turn causes athird class to be loaded, and so forth. Thus, progress through thestages of loading, linking, and initializing can differ from class toclass. Further, some embodiments may delay (perform “lazily”) one ormore functions of the loading, linking, and initializing process untilthe class is actually required. For example, resolution of a methodreference may be delayed until a virtual machine instruction invokingthe method is executed. Thus, the exact timing of when the steps areperformed for each class can vary greatly between implementations.

To begin the loading process, the virtual machine 104 starts up byinvoking the class loader 107 which loads an initial class. Thetechnique by which the initial class is specified will vary fromembodiment to embodiment. For example, one technique may have thevirtual machine 104 accept a command line argument on startup thatspecifies the initial class.

To load a class, the class loader 107 parses the class file 200corresponding to the class and determines whether the class file 200 iswell-formed (meets the syntactic expectations of the virtual machine104). If not, the class loader 107 generates an error. For example, inJava the error might be generated in the form of an exception which isthrown to an exception handler for processing. Otherwise, the classloader 107 generates the in-memory representation of the class byallocating the run-time constant pool 304, method code 305, and fieldand method data 306 for the class within the per-class area 303.

In some embodiments, when the class loader 107 loads a class, the classloader 107 also recursively loads the super-classes of the loaded class.For example, the virtual machine 104 may ensure that the super-classesof a particular class are loaded, linked, and/or initialized beforeproceeding with the loading, linking and initializing process for theparticular class.

During linking, the virtual machine 104 verifies the class, prepares theclass, and performs resolution of the symbolic references defined in therun-time constant pool 304 of the class.

To verify the class, the virtual machine 104 checks whether thein-memory representation of the class is structurally correct. Forexample, the virtual machine 104 may check that each class except thegeneric class Object has a superclass, check that final classes have nosub-classes and final methods are not overridden, check whether constantpool entries are consistent with one another, check whether the currentclass has correct access permissions for classes/fields/structuresreferenced in the constant pool 304, check that the virtual machine 104code of methods will not cause unexpected behavior (e.g. making sure ajump instruction does not send the virtual machine 104 beyond the end ofthe method), and so forth. The exact checks performed duringverification are dependent on the implementation of the virtual machine104. In some cases, verification may cause additional classes to beloaded, but does not necessarily require those classes to also be linkedbefore proceeding. For example, assume Class A contains a reference to astatic field of Class B. During verification, the virtual machine 104may check Class B to ensure that the referenced static field actuallyexists, which might cause loading of Class B, but not necessarily thelinking or initializing of Class B. However, in some embodiments,certain verification checks can be delayed until a later phase, such asbeing checked during resolution of the symbolic references. For example,some embodiments may delay checking the access permissions for symbolicreferences until those references are being resolved.

To prepare a class, the virtual machine 104 initializes static fieldslocated within the field and method data 306 for the class to defaultvalues. In some cases, setting the static fields to default values maynot be the same as running a constructor for the class. For example, theverification process may zero out or set the static fields to valuesthat the constructor would expect those fields to have duringinitialization.

During resolution, the virtual machine 104 dynamically determinesconcrete memory address from the symbolic references included in therun-time constant pool 304 of the class. To resolve the symbolicreferences, the virtual machine 104 utilizes the class loader 107 toload the class identified in the symbolic reference (if not alreadyloaded). Once loaded, the virtual machine 104 has knowledge of thememory location within the per-class area 303 of the referenced classand its fields/methods. The virtual machine 104 then replaces thesymbolic references with a reference to the concrete memory location ofthe referenced class, field, or method. In an embodiment, the virtualmachine 104 caches resolutions to be reused in case the sameclass/name/descriptor is encountered when the virtual machine 104processes another class. For example, in some cases, class A and class Bmay invoke the same method of class C. Thus, when resolution isperformed for class A, that result can be cached and reused duringresolution of the same symbolic reference in class B to reduce overhead.

In some embodiments, the step of resolving the symbolic referencesduring linking is optional. For example, an embodiment may perform thesymbolic resolution in a “lazy” fashion, delaying the step of resolutionuntil a virtual machine instruction that requires the referencedclass/method/field is executed.

During initialization, the virtual machine 104 executes the constructorof the class to set the starting state of that class. For example,initialization may initialize the field and method data 306 for theclass and generate/initialize any class instances on the heap 302created by the constructor. For example, the class file 200 for a classmay specify that a particular method is a constructor that is used forsetting up the starting state. Thus, during initialization, the virtualmachine 104 executes the instructions of that constructor.

In some embodiments, the virtual machine 104 performs resolution onfield and method references by initially checking whether thefield/method is defined in the referenced class. Otherwise, the virtualmachine 104 recursively searches through the super-classes of thereferenced class for the referenced field/method until the field/methodis located, or the top-level superclass is reached, in which case anerror is generated.

3. An Updated Type

FIG. 5 illustrates an example of an updated type, in accordance with oneor more embodiments. As illustrated, an updated type 502 includes fields506, methods 514, one or more migration relationships 522, and one ormore conversion functions 524. In one or more embodiments, the updatedtype 502 may include more or fewer components than the componentsillustrated in FIG. 5. As an example, an updated type may include fields506 without including methods 514. Alternatively, an updated type mayinclude methods 514 without including fields 506.

In one or more embodiments, each of fields 506 (such as fields 506 a-b)is associated with a same field name 508 but different field types (suchas field types 510 a-b). Each field 506 may be referred to as adifferent version of a “migrated field.”

As an example, an updated type 502 may include two fields with a samefield name, fd. One of the two fields may be associated with the fieldtype Date. The other of the two fields may be associated with the fieldtype NewDate. The field type Date may include month and year only. Thefield type NewDate may include month, date, and year. Example codeincluding the above-described migrated field may be written as follows:

class Account { Date fd; @MigratedFrom (Date fd, ConversionFunctionPair)NewDate fd; }

In the above example, the line beginning with @MigratedFrom may be usedto identify a migration relationship 522 between Date fd and NewDate fd.Migration relationships 522 are further described below.

In one or more embodiments, each of methods 514 (such as methods 514a-b) is associated with a same method name 516. But each method isassociated with different sets of parameter types (such as parametertypes 518 a-b) and/or different return types (such as return types 520a-b). A set of parameter types includes one or more parameter typescorresponding respectively to one or more parameters that are input to amethod. A return type corresponds to a value returned by a method. Theterm “method descriptor” refers to (a) the method name, (b) the set ofparameter types, and (c) the return type associated with a particularmethod. Each method, which is associated with a same method name but adifferent method descriptor, may be referred to as a different versionof a “migrated method.”

As an example, an updated type includes two methods with a same methodname, getAccess. One of the two methods is associated with one parameterof the parameter type byte. The method is further associated with areturn type of Date. The other of the two methods is associated with twoparameters, of the parameter types char and String respectively. Themethod is further associated with a return type of NewDate. Example codeincluding the above-described migrated method may be written as follows:

class Account { Date getAccess(byte arg1) { ... }; @MigratedFrom (DategetAccess(byte arg1), ParameterConversionFunctionPair,ReturnConversionFunctionPair) NewDate getAccess(char arg1, String arg2){ ... }; }

In the above example, the line beginning with @MigratedFrom may be usedto identify a migration relationship 522 between Date getAccess (bytearg1) and NewDate getAccess (char arg1, String arg2). Migrationrelationships 522 are further described below.

In one or more embodiments, additional and/or alternative migratedmembers may be included in the updated type 502. A member in a type isan inheritable component of the type. A member in a type is inherited bya sub-type, given that there are no access restrictions to the memberbased on access level modifiers. A member in a type may be directlydeclared in the body of the type. Additionally or alternatively, amember may be included in the type via inheritance. Examples of membersinclude a field, a method, a nested class, an interface, and anenumerated type. An updated type 502 is a type that includes one or moremigrated members.

In one or more embodiments, a migration relationship 522 indicates asequence in which each version of a migrated member was created.Additionally or alternatively, a migration relationship 522 identifieswhich particular version of a migrated member was migrated from anotherversion of the migrated member. As an example, a migration relationshipmay indicate that field 506 b was migrated from field 506 a. Anothermigration relationship may indicate that method 514 b was migrated frommethod 514 a.

A migration relationship 522 may be specified in a migration tag that isassociated with one or more versions of a migrated member. As describedabove, example code including a migration tag may be written as follows:

class Account { Date fd; @MigratedFrom (Date fd, ConversionFunctionPair)NewDate fd; }

The line beginning with @MigratedFrom is a migration tag associated withNewDate fd. In the parentheses, the term “Date fd” indicates thatNewDate fd was migrated from Date fd. Also in the parentheses, the term“ConversionFunctionPair” identifies a pair of conversion functions. Theterm “ConversionFunctionPair” may be, for example, a reference to a pairof conversion functions. Conversion functions are further describedbelow.

Also described above, example code including a migration tag may bewritten as follows:

class Account { Date getAccess (byte arg1) { ... }; @MigratedFrom (DategetAccess(byte arg1), ParameterConversionFunctionPair,ReturnConversionFunctionPair) NewDate getAccess(char arg1, String arg2){ ... } }

The line beginning with @MigratedFrom is a migration tag associated withNewDate getAccess (char arg1, String arg2). In the parentheses, the term“Date getAccess (byte arg1)” indicates that NewDate getAccess (chararg1, String arg2) was migrated from Date getAccess (byte arg1). Also inthe parentheses, the term “ParameterConversionFunctionPair” identifies apair of conversion functions for parameter types of the migrated method.The term “ReturnConversionFunctionPair” identifies a pair of conversionfunctions for return types of the migrated method. Conversion functionsare further described below.

In an embodiment, a migrated member may be associated with three or moreversions. Migration relationships between the different versions may bespecified using multiple migration tags. As an example, a migratedmember may be a field named fd. The versions associated with themigrated member may be: int fd; Date fd; NewDate fd. One migration tagassociated with Date fd may indicate that Date fd was migrated from intfd. Another migration tag associated with NewDate fd may indicate thatNewDate fd was migrated from Date fd.

Additionally or alternatively, a migration relationship 522 may bespecified in a file and/or database that is separate from the fileincluding the updated type 502. As an example, a separate file may storea version list. The version list may be a sequenced list of versions ofa migrated member. As an example, a sequenced list may include: int fd;Date fd; NewDate fd. The sequenced list may indicate that int fd is theearliest version of the migrated field fd; Date fd is a subsequentversion of the migrated field fd; and NewDate fd is the current versionof the migrated field fd.

In one or more embodiments, a conversion function 524 converts values ofa particular set of types into values of another set of types.

As an example, a conversion function may convert a double value into anint value. The double type may be a signed 64-bit two's complementinteger, while the int type may be a signed 32-bit two's complementinteger. The range of the int type may be −2,147,483,648 to+2,147,483,647. When converting a double value into an int value, thedouble value is compared to the range of the int type. If the doublevalue is within the range of the int type, then the least significant32-bits of the double value may be determined as the converted intvalue. If the double value is above the maximum value of the int type,then the maximum value of the int type (+2,147,483,647) may bedetermined as the converted int value. If the double value is below theminimum value of the int type, then the minimum value of the int type(−2,147,483,648) may be determined as the converted int value.

As another example, a conversion function may convert an int value intoa double value. If the int value is a positive number, then theconversion function may pad thirty-two zeroes in front of the int value.If the int value is a negative number, then the conversion function maypad thirty-two ones in front of the int value. The padded int value maybe determined as the converted double value.

A pair of conversion functions includes a projection function and anembedding function. The embedding function performs a reverse conversionas compared to the projection function. The embedding function thatconverts a value of a first type into a value of a second type.Conversely, the projection function that converts a value of the firsttype into a value of the second type. However, the embedding functionand the projection function may not be exact inverses of each other. Theembedding function and the projection function have the followingproperties, as described below.

Given a first value of the first type, an embedding function may beapplied to the first value. A projection function may be applied to theresult of the embedding function. The result of this application of theprojection function is the first value itself.

Given a second value of the second type, a projection function may beapplied to the second value. An embedding function may be applied to theresult of the projection function. The result of this application of theembedding function is either the second value itself, or a value similarand/or related to the second value.

The following example illustrates the above properties of an embeddingfunction and a projection function. As an example, an embedding functionmay convert an int value into a double value. A projection function mayconvert a double value into an int value.

An initial int value may be a binary number including thirteen zeroes inthe most significant bits, followed by nineteen ones in the leastsignificant bits (which is 524,287 in decimal). The embedding functionmay be applied to the initial int value. Based on the embeddingfunction, thirty-two zeros may be padded in front of the int value. Theconverted double value may be a binary number including forty-fivezeroes in the most significant bits, followed by nineteen ones in theleast significant bits (which is 524,287 in decimal). The projectionfunction may be applied to the converted double value. Based on theprojection function, the double value may be compared to the range ofthe int type. Since the double value is within the range, the leastsignificant 32-bits of the double value may be determined as theconverted int value. The converted int value may be a binary numberincluding thirteen zeroes in the most significant bits, followed bynineteen ones in the least significant bits (which is 524,287 indecimal). Hence, the output of the projection function is the same asthe initial int value.

Conversely, an initial double value may be a binary number includingthirty zeroes in the most significant bits, and thirty-four ones in theleast significant bits (which is 17,179,869,184 in decimal). Theprojection function may be applied to the initial double value. Based onthe projection function, the double value may be compared to the rangeof the int type. Since the double value is above the maximum value ofthe int type, the maximum value of the int type may be determined as theconverted int value. The converted int value may be one zero in the mostsignificant bit, followed by thirty-one ones in the least significantbits (which is 2,147,483,647 in decimal). The embedding function may beapplied to the converted int value. Based on the embedding function,thirty-two zeroes may be padded in front of the int value. The converteddouble value may be a binary number including thirty-three zeroes in themost significant bits, followed by thirty-one ones in the leastsignificant bits (which is 2,147,483,647 in decimal). The output of theembedding function is different from the initial double value. Hence,the output of the embedding function is not the same as the initialdouble value. The output of the embedding function is related to theinitial double value, in that (a) the initial double value exceeds themaximum value of the int type, and (b) the output of the embeddingfunction is the maximum value of the int type.

In an embodiment, a conversion function converts a set of values of aparticular set of types into another set of values of another set oftypes. The number of values in the input of a conversion function may bethe same as or different from the number of values in the output of aconversion function. As an example, a conversion function may convert(a) an int value into (b) a double value and a String value. If the intvalue is a positive number, thirty-two zeroes may be padded in front ofthe int value to obtain the converted double value. If the int value isa negative number, thirty-two ones may be padded in front of the intvalue to obtain the converted double value. Additionally, the conversionfunction may set the String value to null. The output of the conversionfunction includes both the converted double value and the String valuethat is set to null.

In an embodiment, a chain of conversion functions may be used to converta value of one type into a value of another type. As described above, amigrated member may be associated with three or more versions. Oneconversion function may convert between two particular versions. Anotherconversion function may convert between two other versions. The twoconversion functions may be chained together. As an example, a migratedmember may be a field named fd. The versions associated with themigrated member may be: int fd; Date fd; NewDate fd. A first conversionfunction may convert an int value into a Date value. A second conversionfunction may convert a Date value into a NewDate value. To convert aparticular int value into a NewDate value, the first conversion functionmay be applied to the particular int value, and the second conversionfunction may be applied to the result of the first conversion function.Hence, the first and second conversion functions are chained together toconvert an int value into a NewDate value.

4. Compiling an Updated Type

FIG. 6 illustrates a set of operations for compiling an updated typecomprising an migrated member, in accordance with one or moreembodiments. One or more operations illustrated in FIG. 6 may bemodified, rearranged, or omitted all together. Accordingly, theparticular sequence of operations illustrated in FIG. 6 should not beconstrued as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a source file, anupdated type including multiple versions of a migrated member (Operation602). Identifying the updated type may be performed as part of a processto compile the updated type.

In an embodiment, the updated type includes multiple versions of amigrated field, each associated with a same field name but differentfield types, as explained above with reference to fields 506 in FIG. 5.Additionally or alternatively, the updated type includes multipleversions of a migrated method, each associated with a same method namebut different parameter types and/or different return types, asexplained above with reference to methods 514 in FIG. 5.

One or more embodiments include identifying one or more migrationrelationships between the versions of the migrated member (Operation604). A migration relationship is identified based on one or moremigration tags that are associated with one or more versions of themigrated member. The migration tag indicates a sequence in which theversions of the migrated member were generated. Additionally oralternatively, the migration relationship is identified from a separatefile and/or database.

One or more embodiments include determining whether the migrations arevalid (Operation 606).

In an embodiment, a migration may be invalid if a version of a migratedmember was associated with a security setting that prohibited migration.For example, a version of the migrated member may be marked with an“Unmigratable” tag.

In an embodiment, a migration may be invalid if different versions of amigrated member are associated with conflicting migration relationships.As an example, an updated type may include two fields with a same fieldname, fd. One of the two fields may be associated with the type Date.The other of the two fields may be associated with the type NewDate. Thefield NewDate fd may be associated with a migration tag that indicatesNewDate fd was migrated from Date fd. Meanwhile, the field Date fd maybe associated with a migration tag that indicates Date fd was migratedfrom NewDate fd. The two migration tags indicate conflicting migrationrelationships. The migration is invalid.

If the migration is not valid, then an error is returned (Operation608). The source file is not successfully compiled.

One or more embodiments include identifying one or more pairs ofconversion functions (Operation 610). The pairs of conversion functionsmay be used to convert between the different versions of the migratedmember. References to the conversion functions are identified from thesource file including the updated type. As an example, references toconversion functions may be identified from a migration tag associatedwith a version of a migrated member. Based on the references to theconversion functions, the conversion functions may be identified fromthe source file itself or a different file.

One or more embodiments include storing, in a class file, a set ofmember structures corresponding to the versions of the migrated member(Operation 612).

In an embodiment, the updated type includes multiple fields, eachassociated with a same field name. The multiple fields are differentversions of a migrated field. Compilation of the multiple fieldsassociated with the same field name does not cause a generation of anerror. Rather, a set of field structures, corresponding to the versionsof the migrated field, are stored in the compiled class file.

In an embodiment, the updated type includes multiple methods, eachassociated with a same method name but different parameter types and/orreturn types. The multiple methods are different versions of a migratedmethod. A set of method structures, corresponding to the versions of themigrated method, are stored in the compiled class file. As an example,an updated type may include multiple methods associated with a samemethod signature. The term “method signature” refers to (a) the methodname of a method, and (b) the set of parameter types associated with themethod. The methods may be associated with different return types. Themethods are different versions of a migrated method. Compilation of themultiple methods associated with the same method signature does notcause a generation of an error. Rather, a set of method structures,corresponding to the versions of the migrated method, may be stored in acompiled class file.

One or more embodiments include storing, in the class file, themigration relationships (Operation 614). The migration relationships maybe stored as migration tags in the class file. Additionally oralternatively, the migration relationships may be stored as a versionlist in the class file. Additionally or alternatively, other formats maybe used for storing the migration relationships.

One or more embodiments include storing, in the class file, referencesto the pairs of conversion functions (Operation 616). The references tothe pairs of conversion functions may be stored in a constant pool tableof the class file. Additionally or alternatively, other formats may beused for storing the references to the pairs of conversion functions.

In an embodiment, the conversion functions themselves may be stored inthe class file. In another embodiment, the conversion functions arestored in a separate file and/or database.

5. Accessing a Migrated Field

FIGS. 7A-B illustrate a set of operations for fetching and returning avalue of a migrated field as a value of a particular type, in accordancewith one or more embodiments. One or more operations illustrated inFIGS. 7A-B may be modified, rearranged, or omitted all together.Accordingly, the particular sequence of operations illustrated in FIGS.7A-B should not be construed as limiting the scope of one or moreembodiments.

One or more embodiments include identifying, in a class file, one ormore instructions to fetch and return (a) a value of a migrated fieldcorresponding to an object as (b) a value of a particular type(Operation 704). Identifying the instructions in the class file may beperformed as part of a process to execute the class file. In anembodiment, the instructions are identified from the body of a mainmethod. The main method is a method that is initially called by theruntime environment. In another embodiment, the instructions areidentified from the body of another method that is called by the mainmethod.

Example code may be written as follows:

class Main { public static void main(String[ ] args) { Account myaccount= new Account( ); int myfd = myaccount.fd; } }

The example code may be compiled into bytecode, which may be stored in aclass file. The instructions to fetch and return (a) a value of amigrated field corresponding to an object as (b) a value of a particulartype include the bytecode corresponding to the line int my fd=myaccount.fd. The bytecode corresponding to the line intmyfd=myaccount.fd may include, for example, a getfield instruction.Based on the line int myfd =myaccount.fd, a value of a particular field,fd, corresponding to the object, myaccount, is to be fetched. The valueof the particular field, fd, is to be returned as a value of theparticular type, int. The particular field, fd, may subsequently bedetermined as a migrated field by identifying an updated type thatincludes the particular field, as further described below with referenceto Operation 706.

One or more embodiments include identifying multiple versions of themigrated field, each associated with a same field name but differentfield types (Operation 706).

An updated type including the migrated field is identified. The updatedtype is identified based on a declaration of the object corresponding tothe migrated field. The declaration of the object indicates a typeassociated with the object. The type associated with the object is theupdated type including the migrated field.

Referring to the example code above, the migrated field, fd, correspondsto the object, myaccount. A declaration of myaccount is the line Accountmyaccount=new Account( ). Based on the declaration of myaccount, thetype of myaccount is determined as Account. Account is the updated typeincluding the migrated field, fd.

If the updated type is not yet loaded, then the updated type is loadedinto a per-class area of the runtime environment.

After loading the updated type, multiple versions of the migrated fieldare identified from the updated type. Each version of the migrated fieldis associated with a same field name but different field types. As anexample, Account may be an updated type including the migrated field,fd. The updated type Account may include the following code:

class Account { int fd; @MigratedFrom (int fd, ConversionFunctionPair)long fd; }

The example code for the updated type, Account, includes multiple fieldsassociated with the field name, fd. One fd field is associated with thefield type int. Another fd field is associated with the field type long.The two fd fields are different versions of a migrated field. Theversions of the migrated field are identified from the updated typeAccount.

One or more embodiments include determining one or more migrationrelationships between the versions of the migrated field (Operation708). The migration relationships between the versions of the migratedfield are stored as part of the update type. The migration relationshipsare identified from the updated type. Additionally or alternatively, themigration relationships are identified based on references stored aspart of the updated type.

Based on the migration relationships, a sequence in which the versionsof the migrated field were generated is determined. A current version ofthe migrated field is determined.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 704, is the sameas a field type associated with any of the versions of the migratedfield (Operation 710). The particular type, specified in theinstructions identified at Operation 704, is compared to each of thefield types associated with the versions of the migrated field.

If the particular type is not the same as a field type associated withany of the versions of the migrated field, then an error is returned(Operation 712). The class file is not successfully executed.

One or more embodiments include fetching a value associated with acurrent version of the migrated field corresponding to the object(Operation 714). As discussed above with reference to Operation 708, thesequence in which the versions of the migrated field were generated isdetermined based on the migration relationships. The version that waslast generated is the current version of the migrated field.

As described above, instructions to fetch a value of the migrated fieldcorresponding to the object are identified at Operation 704. In anembodiment, the instructions include a symbolic reference to themigrated field. During linkage of the class file, the symbolic referenceto the migrated field is resolved as a direct reference to the currentversion of the migrated field.

As an example, a class file may include a symbolic reference to amigrated field, fd. An updated type, Account, including the migratedfield may include two versions of fd: int fd and long fd. Based on amigration relationship between the two versions, long fd may beidentified as a current version of the migrated field. The symbolicreference to fd may be resolved as a direct reference to long fd.

Based on the resolved direct reference to the current version of themigrated field, attributes of the current version of the migrated fieldare identified from the updated type. One of the attributes includes anoffset, with respect to a memory location of an object of the updatedtype, associated with the current version of the migrated field. The sumof the memory location of the object of the updated type and the offsetis a memory location of a value of the current version of the migratedfield corresponding to the object.

During execution of the class file, a memory location of the object,corresponding to the migrated field, is identified. A sum of (a) thememory location of the object and (b) the offset is computed. Asdescribed above, the sum is determined as a memory location of a valueof the current version of the migrated field corresponding to theobject. A value stored at the determined memory location is fetched.

Additional and/or alternative methods for determining a memory locationof a value of the current version of the migrated field may be used. Avalue stored at the determined memory location is fetched.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 704, is the sameas the field type associated with the current version of the migratedfield (Operation 716). As described above, based on the resolved directreference to the current version of the migrated field, attributes ofthe current version of the migrated field are identified from theupdated type. One of the attributes includes a field type associatedwith the current version of the migrated field. Hence, the field typeassociated with the current version of the migrated field is determinedfrom the updated type. Additional and/or alternative methods fordetermining a field type associated with the current version of themigrated field may be used.

The particular type, specified in the instructions identified atOperation 704, is compared to the field type associated with the currentversion of the migrated field.

If the particular type, specified in the instructions identified atOperation 704, is the same as the field type associated with the currentversion of the migrated field, then the fetched value is returned(Operation 718). The fetched value is stored as the value of theparticular type, as specified in the instructions identified atOperation 704.

One or more embodiments include identifying one or more conversionfunctions that convert (a) a value of the field type associated with thecurrent version of the migrated field into (b) a value of the particulartype (Operation 720). The conversion function may be identified from theupdated type, the class file identified at Operation 704, a separatefile, and/or a separate database. As an example, an updated type mayinclude a migrated field and a migration tag associated with themigrated field. The migration tag may identify a reference to aconversion function. The reference may be an index to a constant poolentry in the updated type. The conversion function may be identifiedfrom the constant pool table based on the index.

One or more embodiments include applying the conversion functions toconvert (a) the fetched value into (b) a value of the particular type(Operation 722).

A single conversion function may be used to convert (a) the fetchedvalue into (b) a value of the particular type, as described above withreference to conversion functions 524 of FIG. 5. Alternatively, multipleconversion functions may be chained together to convert (a) the fetchedvalue into (b) a value of the particular type, as described above withreference to conversion functions 524 of FIG. 5.

One or more embodiments include returning the converted value (Operation724). The converted value is stored as the value of the particular type,as specified in the instructions identified at Operation 704.

FIGS. 8A-B illustrate a set of operations for storing a value of aparticular type as a value of a migrated field, in accordance with oneor more embodiments. One or more operations illustrated in FIGS. 8A-Bmay be modified, rearranged, or omitted all together. Accordingly, theparticular sequence of operations illustrated in FIGS. 8A-B should notbe construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file, one ormore instructions to store (a) a value of a particular type as (b) avalue of a migrated field corresponding to an object (Operation 804).Identifying the instructions in the class file may be performed as partof a process to execute the class file. Further descriptions regardingthe operation of identifying instructions are included above withreference to Operation 704.

Example code may be written as follows:

class Main { public static void main(String[ ] args) { Account myaccount= new Account( ); int myfd = 10; myaccount.fd = myfd; } }

The example code may be compiled into bytecode, which may be stored in aclass file. The instructions to store (a) a value of a particular typeas (b) a value of a migrated field corresponding to an object includethe bytecode corresponding to the line myaccount.fd=myfd. The bytecodecorresponding to the line myaccount.fd=myfd may include, for example, aput field instruction. Based on the line myaccount.fd=myfd, a value ofthe particular type, int, is to be stored as a value of the particularfield, fd, corresponding to the object, myaccount. The particular field,fd, may subsequently be determined as a migrated field by identifying anupdated type that includes the particular field, as further describedbelow with reference to Operation 806.

One or more embodiments include identifying multiple versions of themigrated field, each associated with a same field name but differentfield types (Operation 806). Further descriptions regarding theoperation of identifying multiple versions of the migrated field areincluded above with reference to Operation 706.

One or more embodiments include determining one or more migrationrelationships between the versions of the migrated field (Operation808). Further descriptions regarding the operation of determiningmigration relationships are included above with reference to Operation708.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 804, is the sameas a field type associated with any of the versions of the migratedfield (Operation 810). The particular type, specified in theinstructions identified at Operation 804, is compared to each of thefield types associated with the versions of the migrated field.

If the particular type is not the same as a field type associated withany of the versions of the migrated field, then an error is returned(Operation 812). The class file is not successfully executed.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 804, is the sameas the field type associated with the current version of the migratedfield (Operation 814).

As described above, instructions to store a value of the particular typeas a value of the migrated field corresponding to the object areidentified at Operation 804. In an embodiment, the instructions includea symbolic reference to the migrated field. During linkage of the classfile, the symbolic reference to the migrated field is resolved as adirect reference to the current version of the migrated field. Furtherdescriptions regarding the operation of resolving a symbolic referenceas a direct reference are included above with reference to Operation714.

Based on the resolved direct reference to the current version of themigrated field, attributes of the current version of the migrated fieldare identified from the updated type. One of the attributes includes afield type associated with the current version of the migrated field.Hence, the field type associated with the current version of themigrated field is determined from the updated type. Additional and/oralternative methods for determining a field type associated with thecurrent version of the migrated field may be used. Further descriptionsregarding determining a field type associated with the current versionof the migrated field are included above with reference to Operation716.

The particular type, specified in the instructions identified atOperation 804, is compared to the field type associated with the currentversion of the migrated field.

If the particular type is the same as the field type associated with thecurrent version of the migrated field, then the value of the particulartype, specified in the instructions identified at Operation 804, isstored as a value of the current version of the migrated fieldcorresponding to the object (Operation 816).

As described above, based on the resolved direct reference to thecurrent version of the migrated field, attributes of the current versionof the migrated field are identified from the updated type. One of theattributes includes an offset, with respect to a memory location of anobject of the updated type, associated with the current version of themigrated field. The sum of the memory location of the object of theupdated type and the offset is a memory location of a value of thecurrent version of the migrated field corresponding to the object.

During execution of the class file, a memory location of the object,corresponding to the migrated field, is identified. The sum of (a) thememory location of the object and (b) the offset is computed. Asdescribed above, the sum is determined as a memory location of a valueof the current version of the migrated field corresponding to theobject. The value of the particular type, specified in the instructionsidentified at Operation 804, is stored into the determined memorylocation.

Additional and/or alternative methods for determining a memory locationof a value of the current version of the migrated field may be used.Further descriptions regarding determining a memory location of a valueof the current version of the migrated field are included above withreference to Operation 714. The value of the particular type, specifiedin the instructions identified at Operation 804, is stored into thedetermined memory location.

One or more embodiments include identifying one or more conversionfunctions that convert (a) a value of the particular type into (b) avalue of the field type associated with the current version of themigrated field (Operation 818). Further descriptions regarding theoperation of identifying conversion functions are included above withreference to Operation 720.

One or more embodiments include applying the conversion functions toconvert (a) the value of the particular type, specified in theinstructions identified at Operation 804, into (b) a value of the fieldtype associated with the current version of the migrated field(Operation 820). Further descriptions regarding the operation ofapplying conversion functions are included above with reference toOperation 722.

One or more embodiments include storing the converted value as a valueof the current version of the migrated field corresponding to the object(Operation 822). As described above with reference to Operation 816, amemory location of a value of the current version of the migrated fieldcorresponding to the object is determined. The converted value, outputfrom Operation 820, is stored into the determined memory location.

6. Accessing a Migrated Method

FIGS. 9A-B illustrate a set of operations for invoking a method usingvalues of a particular set of types as arguments to the method, inaccordance with one or more embodiments. One or more operationsillustrated in FIGS. 9A-B may be modified, rearranged, or omitted alltogether. Accordingly, the particular sequence of operations illustratedin FIGS. 9A-B should not be construed as limiting the scope of one ormore embodiments.

One or more embodiments include identifying, in a class file, one ormore instructions to invoke a migrated method, corresponding to anobject, using a set of arguments (Operation 904). Identifying theinstructions in the class file may be performed as part of a process toexecute the class file. Further descriptions regarding the operation ofidentifying instructions are included above with reference to Operation704.

Example code may be written as follows:

class Main { public static void main(String[ ] args) { Account myaccount= new Account( ); byte myarg1 = 3; int dt = myaccount.getAccess(myarg1);} }

The example code may be compiled into bytecode, which may be stored in aclass file. The instructions to invoke the migrated method,corresponding to the object, using the set of arguments include thebytecode corresponding to the line int dt=myaccount.getAccess (myarg1).The bytecode corresponding to the line int dt=myaccount.getAccess(myarg1) may include, for example, a load instruction and an invokeinstruction. Based on the line int dt=myaccount.getAccess (myarg1), anargument of a particular type, byte, is loaded onto an operand stack.Further, a particular method, getAccess, corresponding to the object,myaccount, is executed using the loaded value as an argument. Theparticular method, getAccess, may subsequently be determined as amigrated method by identifying an updated type that includes theparticular method, as further described below with reference toOperation 906.

One or more embodiments include identifying multiple versions of themigrated method, each associated with a same method name but differentsets of parameter types (Operation 906).

An updated type including the migrated method is identified. The updatedtype is identified based on a declaration of the object corresponding tothe migrated method. The declaration of the object indicates a typeassociated with the object. The type associated with the object is theupdated type including the migrated method.

Referring to the example code above, the migrated method, getAccess,corresponds to the object, myaccount. A declaration of myaccount is theline Account myaccount=new Account( ). Based on the declaration ofmyaccount, the type of myaccount is determined as Account. Account isthe updated type including the migrated method, getAccess.

If the updated type is not yet loaded, then the updated type is loadedinto a per-class area of the runtime environment.

After loading the updated type, multiple versions of the migrated methodare identified from the updated type. Each version of the migratedmethod is associated with a same method name but different sets ofparameter types and/or different return types. As an example, Accountmay be an updated type including the migrated method, getAccess. Theupdated type Account may include the following code:

class Account { int getAccess (byte arg1); @MigratedFrom (intgetAccess(byte arg1), ParameterConversionFunctionPair) int getAccess(char arg1, String arg2); }

The example code for the updated type, Account, includes multiplemethods associated with the method name, getAccess. One getAccess methodis associated with the set of parameter types, (byte). Another getAccessmethod is associated with the set of parameter types, (char, String).The two getAccess methods are different versions of a migrated method.The versions of the migrated method are identified from the updated typeAccount.

One or more embodiments include determining one or more migrationrelationships between the versions of the migrated method (Operation908). Further descriptions regarding the operation of determiningmigration relationships are included above with reference to Operation708.

One or more embodiments include determining whether a set of types,associated with the set of arguments, is the same as a set of parametertypes associated with any of the versions of the migrated method(Operation 910). The set of types, associated with the set of arguments,is compared to each of the set of parameter types associated with theversions of the migrated method.

If the set of types, associated with the set of arguments, is not thesame as a set of parameter types associated with any of the versions ofthe migrated method, then an error is returned (Operation 912). Theclass file is not successfully executed.

One or more embodiments include determining whether the set of types,associated with the set of arguments, is the same as the set ofparameter types associated with the current version of the migratedmethod (Operation 914). Based on the migration relationships determinedat Operation 908, the sequence in which the versions of the migratedmethod were created is determined. The version that was last created isthe current version of the migrated method.

The set of types, associated with the set of arguments specified in theinstructions identified at Operation 904, is compared to the set ofparameter types associated with the current version of the migratedmethod.

If the set of types, associated with the set of arguments, is the sameas the set of parameter types associated with the current version of themigrated method, then the current version of the migrated method isexecuted using the set of arguments specified in the instructionsidentified at Operation 904 (Operation 916).

As described above, instructions to invoke the migrated method,corresponding to the object, are identified at Operation 904. In anembodiment, the instructions include a symbolic reference to themigrated method. During linkage of the class file, the symbolicreference to the migrated method is resolved as a direct reference tothe current version of the migrated method. The direct reference to thecurrent version of the migrated method is the memory location of thefirst instruction of the current version of the migrated method.

During execution of the class file, the set of arguments, specified inthe instructions identified at Operation 904, are loaded onto an operandstack. The program counter (PC), which references the next instructionto be executed, is moved to the direct reference to the current versionof the migrated method. Based on the PC, the current version of themigrated method is executed. Further, the loaded values on the operandstack are used as arguments for the current version of the migratedmethod.

Additional and/or alternative methods for identifying the memorylocation associated with the current version of the migrated method maybe used. Additional and/or alternative methods for inputting and/orloading the set of arguments for the current version of the migratedmethod may be used.

One or more embodiments include identifying one or more conversionfunctions that convert (a) values of the set of types associated withthe set of arguments into (b) values of the set of parameter typesassociated with the current version of the migrated method (Operation918). Further descriptions relating to identifying conversion functionsare included above with reference to Operation 720.

In an embodiment, a separate conversion function is used for eachparameter of a migrated method. As an example, a migrated method mayhave the parameters arg1 and arg2. One version of the migrated methodmay specify that arg1 is of type int, and arg2 is of type char. Anotherversion of the migrated method may specify that arg1 is of type double,and arg2 is of type long. Two separate conversion functions may beassociated with arg1 and arg2, respectively. Specifically, a firstconversion function, associated with arg1, may convert an int value intoa double value. A second conversion function, associated with arg2, mayconvert a char value into a long value.

In an embodiment, a single conversion function is used for a set ofparameters of a migrated method. As an example, a migrated method mayhave the parameters arg1 and arg2. One version of the migrated methodmay specify that arg1 is of type int, and arg2 is of type char. Anotherversion of the migrated method may specify that arg1 is of type double,and arg2 is of type long. A single conversion function may convert a setof values comprising an int value and a char value into another set ofvalues comprising a double value and a long value.

One or more embodiments include applying the conversion functions toconvert (a) the set of arguments into (b) values of the set of parametertypes associated with the current version of the migrated method(Operation 920). Further descriptions relating to applying conversionfunctions are included above with reference to Operation 722.

In an embodiment, as described above, a separate conversion function isused for each parameter of a migrated method. Hence, a separateconversion function is applied to each of the set of arguments. Theconverted values, output from each conversion function, are input asarguments to the current version of the migrated method.

In an embodiment, as described above, a single conversion function isused for a set of parameters of a migrated method. Hence, the singleconversion function is applied to the set of arguments. The convertedvalues, output from the single conversion function, are input asarguments to the current version of the migrated method.

One or more embodiments include executing the current version of themigrated method using the converted values (Operation 922). As describedabove with reference to Operation 916, a memory location associated withthe current version of the migrated method is identified. The currentversion of the migrated method is executed from the identified memorylocation. Further, the converted values, output from Operation 920, areloaded as arguments for the current version of the migrated method. Thecurrent version of the migrated method is executed using the convertedvalues.

FIGS. 10A-B illustrate a set of operations for returning a value from amethod as a value of a particular type, in accordance with one or moreembodiments. One or more operations illustrated in FIGS. 10A-B may bemodified, rearranged, or omitted all together. Accordingly, theparticular sequence of operations illustrated in FIGS. 10A-B should notbe construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file,instructions to invoke a migrated method, corresponding to an object,and to return a value from the migrated method as a value of aparticular type (Operation 1004). Identifying the instructions in theclass file may be performed as part of a process to execute the classfile. Further descriptions regarding the operation of identifyinginstructions are included above with reference to Operation 704.

Example code may be written as follows:

class Main { public static void main(String[ ] args) { Account myaccount= new Account( ); byte myarg1 = 3; int dt = myaccount.getAccess(myarg1);} }

The example code may be compiled into bytecode, which may be stored in aclass file. The instructions to invoke the migrated method,corresponding to the object, and to return a value from the migratedmethod as a value of the particular type include the bytecodecorresponding to the line int dt=myaccount.getAccess (myarg1). Thebytecode corresponding to the line int dt=myaccount.getAccess (myarg1)may include, for example, an invoke instruction and a store instruction.Based on the line int dt=myaccount.getAccess (myarg1), a particularmethod, getAccess, corresponding to the object, myaccount, is executed.A value returned from the particular method is stored as a value of theparticular type, int. The particular method, getAccess, may subsequentlybe determined as a migrated method by identifying an updated type thatincludes the particular method, as further described below withreference to Operation 1006.

One or more embodiments include identifying multiple versions of themigrated method, each associated with a same method name but differentreturn types (Operation 1006). Further descriptions relating toidentifying an updated type including the migrated method are includedabove with reference to Operation 906. Multiple versions of the migratedmethod are identified from the updated type. Each version of themigrated method is associated with a same method name but different setsof parameter types and/or different return types. As an example, Accountmay be an updated type including the migrated method, getAccess. Theupdated type Account may include the following code:

class Account { int getAccess (byte arg1); @MigratedFrom (intgetAccess(byte arg1), ReturnConversionFunctionPair) long getAccess (bytearg1); }

The example code for the updated type, Account, includes multiplemethods associated with the method name, getAccess. One getAccess methodis associated with the return type, int. Another getAccess method isassociated with the return type, long. The two getAccess methods aredifferent versions of a migrated method. The versions of the migratedmethod are identified from the updated type Account.

One or more embodiments include determining one or more migrationrelationships between the versions of the migrated methods (Operation1008). Further descriptions regarding the operation of determiningmigration relationships are included above with reference to Operation708.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 1004, is the sameas a return type associated with any of the versions of the migratedmethod (Operation 1010). The particular type, specified in theinstructions identified at Operation 1004, is compared to each of thereturn types associated with the versions of the migrated method.

If the particular type, specified in the instructions identified atOperation 1004, is not the same as a return type associated with any ofthe versions of the migrated method, then an error is returned(Operation 1012). The class file is not successfully executed.

One or more embodiments include executing the current version of themigrated method (Operation 1014).

As described above with reference to Operation 916, a memory locationassociated with the current version of the migrated method isidentified. The current version of the migrated method is executed fromthe identified memory location.

One or more embodiments include determining whether the particular type,specified in the instructions identified at Operation 1004, is the sameas the return type associated with the current version of the migratedmethod (Operation 1016). The particular type, specified in theinstructions identified at Operation 1004, is compared to the returntype associated with the current version of the migrated method.

If the particular type, specified in the instructions identified atOperation 1004, is the same as the return type associated with thecurrent version of the migrated method, then a value is returned fromthe current version of the migrated method (Operation 1018). The valuereturned from the current version of the migrated method is returned asa value of the particular type, as specified in the instructionsidentified at Operation 1004.

One or more embodiments include identifying one or more conversionfunctions that convert (a) a value of the return type associated withthe current version of the migrated method into (b) a value of theparticular type (Operation 1020). Further descriptions relating toidentifying conversion functions are included above with reference toOperation 720.

One or more embodiments include applying the conversion functions toconvert (a) a value returned from the current version of the migratedmethod into (b) a value of the particular type (Operation 1022). Furtherdescriptions relating to applying conversion functions are includedabove with reference to Operation 722.

One or more embodiments include returning the converted value (Operation1024). The converted value is returned as a value of the particulartype, as specified in the instructions identified at Operation 1004.

7. Example Embodiments EXAMPLE 1 Storing a Value of a Particular Type asa Value of a Migrated Field

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, causes performance ofoperations comprising:

-   identifying one or more instructions to store (a) a first value of a    first type as (b) a second value of a first field corresponding to a    particular object, wherein the first field is in an updated type and    referenced by a particular field name;-   determining that the first field is associated with a second type    different than the first type;-   responsive to determining that the updated type comprises a second    field associated with (a) the particular field name and (b) the    first type:-   executing a conversion function to convert the first value of the    first type into a third value of the second type; and-   storing the third value as the second value of the first field.

EXAMPLE 2

The medium of EXAMPLE 1, wherein executing the conversion function isfurther responsive to: determining that the first field was migrated tothe second field.

EXAMPLE 3 Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, cause performance ofoperations comprising:

-   identifying, in a set of source code, a first declaration for a    first method associated with (a) a particular method name, and (b) a    first set of parameter types, the first set of parameter types    comprising one or more parameter types;-   identifying, in the set of source code, a second declaration for a    second method associated with (a) the particular method name,    and (b) a second set of parameter types different than the first set    of parameter types;-   determining that the first method was migrated to the second method;-   storing, in a set of compiled code, a first method structure    associated with (a) the particular method name, and (b) the first    set of parameter types;-   storing, in the set of compiled code, a second method structure    associated with (a) the particular method name, and (b) the second    set of parameter types; and-   storing, in the set of compiled code, a migration relationship    indicating that the first method was migrated to the second method.

EXAMPLE 4

The medium of EXAMPLE 3, wherein the operations further comprise:

-   identifying, in the set of source code, a first reference to a    conversion function whose effect is to convert one or more values of    the first set of parameter types into one or more values of the    second set of parameter types; and-   storing, in the set of compiled code, a second reference to the    conversion function.

EXAMPLE 5

The medium of EXAMPLE 3, wherein the operations further comprise:identifying, in the set of source code, a first reference to aconversion function that converts a value of a first parameter type, ofthe first set of parameter types, into a value of a second parametertype, of the second set of parameter types; and storing, in the set ofcompiled code, a second reference to the conversion function.

EXAMPLE 6

The medium of EXAMPLE 3, wherein:

-   the first method is further associated with a first return type;-   the second method is further associated with a second return type    different than the first return type;-   the first method structure is further associated with the first    return type;-   the second method structure is further associated with the second    return type;-   the operations further comprise:    -   identifying, in the set of source code:        -   a first reference to a first conversion function whose            effect is to convert one or more values of the first set of            parameter types into one or more values of the second set of            parameter types;        -   a second reference to a second conversion function that            converts a value of the first return type into a value of            the second return type;    -   storing, in the set of compiled code:        -   a third reference to the first conversion function; and        -   a fourth reference to the second conversion function.

EXAMPLE 7 Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, cause performance ofoperations comprising:

-   identifying, in a set of source code, a first declaration for a    first method associated with (a) a particular method name, and (b) a    first return type;-   identifying, in the set of source code, a second declaration for a    second method associated with (a) the particular method name,    and (b) a second return type different than the first return type;-   determining that the first method was migrated to the second method;-   storing, in a set of compiled code, a first method structure    associated with (a) the particular method name, and (b) the first    return type;-   storing, in the set of compiled code, a second method structure    associated with (a) the particular method name, and (b) the second    return type; and-   storing, in the set of compiled code, a migration relationship    indicating that the first method was migrated to the second method.

EXAMPLE 8

The medium of EXAMPLE 7, wherein the operations further comprise:

-   identifying, in the set of source code, a first reference to a    conversion function that converts a value of the first return type    into a value of the second return type; and-   storing, in the set of compiled code, a second reference to the    conversion function.

EXAMPLE 9

The medium of EXAMPLE 7, wherein the operations further comprise:identifying, in the set of source code, a first reference to aconversion function that converts a value of the second return type intoa value of the first return type; and storing, in the set of compiledcode, a second reference to the conversion function.

EXAMPLE 10 Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, cause performance ofoperations comprising:

-   identifying, in a set of source code, a first declaration for a    first method associated with (a) a particular method name, (b) a set    of one or more parameter types, and (c) a first return type;-   identifying, in the set of source code, a second declaration for a    second method associated with (a) the particular method name, (b)    the set of parameter types, and (c) a second return type different    than the first return type;-   storing, in a set of compiled code, a first method structure    associated with (a) the particular method name, (b) the set of    parameter types, and (c) the first return type; and-   storing, in the set of compiled code, a second method structure    associated with (a) the particular method name, (b) the set of    parameter types, and (c) the second return type.

EXAMPLE 11

The medium of EXAMPLE 10, wherein the operations further comprise:

refraining from generating a compilation error, during compilation ofthe set of source code to generate the set of compiled code, based onthe set of source code comprising both the first declaration and thesecond declaration.

EXAMPLE 12 Invoking a Migrated Method Using Arguments of a ParticularSet of Types

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, cause performance ofoperations comprising:

-   identifying one or more instructions to invoke a particular method    corresponding to a particular object, using values corresponding to    a particular set of types as arguments, wherein the particular    method is in an updated type and referenced by a particular method    name;-   determining that the updated type comprises:    -   a first method associated with the particular method name and a        first set of parameter types; and    -   a second method associated with the particular method name and a        second set of parameter types different than the first set of        parameter types;-   responsive to determining that the particular set of types is same    as the first set of parameter types:    -   executing a conversion function to convert the values        corresponding to the particular set of types into values        corresponding to the second set of parameter types; and    -   invoking the second method using the values corresponding to the        second set of parameter types as the arguments.

EXAMPLE 13

The medium of EXAMPLE 12, wherein executing the conversion function isfurther responsive to determining that the first method was migrated tothe second method.

EXAMPLE 14 Returning a Value from a Migrated Method as a Value of aParticular Type

A non-transitory computer readable medium comprising instructions, whichwhen executed by one or more hardware processors, cause performance ofoperations comprising:

-   identifying one or more instructions to return a first value from a    particular method as a second value of a particular type, wherein    the particular method is in an updated type and referenced by a    particular method name;-   determining that the updated type comprises:    -   a first method associated with the particular method name and a        first return type; and    -   a second method associated with the particular method name and a        second return type different than the first return type;-   responsive to determining that the particular type is same as the    first return type:    -   identifying a third value returned from the second method;    -   executing a conversion function to convert the third value into        a fourth value of the particular type; and    -   returning the fourth value as the second value of the particular        type.

EXAMPLE 15

The medium of EXAMPLE 14, wherein executing the conversion function isfurther responsive to determining that the first method was migrated tothe second method.

EXAMPLE 16

The medium of EXAMPLE 14, wherein the one or more instructions comprisean instruction to invoke the particular method.

8. Miscellaneous; Extensions

Embodiments are directed to a system with one or more devices thatinclude a hardware processor and that are configured to perform any ofthe operations described herein and/or recited in any of the claimsbelow.

In an embodiment, a non-transitory computer readable storage mediumcomprises instructions which, when executed by one or more hardwareprocessors, causes performance of any of the operations described hereinand/or recited in any of the claims.

Any combination of the features and functionalities described herein maybe used in accordance with one or more embodiments. In the foregoingspecification, embodiments have been described with reference tonumerous specific details that may vary from implementation toimplementation. The specification and drawings are, accordingly, to beregarded in an illustrative rather than a restrictive sense. The soleand exclusive indicator of the scope of the invention, and what isintended by the applicants to be the scope of the invention, is theliteral and equivalent scope of the set of claims that issue from thisapplication, in the specific form in which such claims issue, includingany subsequent correction.

9. Hardware Overview

According to one embodiment, the techniques described herein areimplemented by one or more special-purpose computing devices. Thespecial-purpose computing devices may be hard-wired to perform thetechniques, or may include digital electronic devices such as one ormore application-specific integrated circuits (ASICs) or fieldprogrammable gate arrays (FPGAs) that are persistently programmed toperform the techniques, or may include one or more general purposehardware processors programmed to perform the techniques pursuant toprogram instructions in firmware, memory, other storage, or acombination. Such special-purpose computing devices may also combinecustom hard-wired logic, ASICs, or FPGAs with custom programming toaccomplish the techniques. The special-purpose computing devices may bedesktop computer systems, portable computer systems, handheld devices,networking devices or any other device that incorporates hard-wiredand/or program logic to implement the techniques.

For example, FIG. 11 is a block diagram that illustrates a computersystem 1100 upon which an embodiment of the invention may beimplemented. Computer system 1100 includes a bus 1102 or othercommunication mechanism for communicating information, and a hardwareprocessor 1104 coupled with bus 1102 for processing information.Hardware processor 1104 may be, for example, a general purposemicroprocessor.

Computer system 1100 also includes a main memory 1106, such as a randomaccess memory (RAM) or other dynamic storage device, coupled to bus 1102for storing information and instructions to be executed by processor1104. Main memory 1106 also may be used for storing temporary variablesor other intermediate information during execution of instructions to beexecuted by processor 1104. Such instructions, when stored innon-transitory storage media accessible to processor 1104, rendercomputer system 1100 into a special-purpose machine that is customizedto perform the operations specified in the instructions.

Computer system 1100 further includes a read only memory (ROM) 1108 orother static storage device coupled to bus 1102 for storing staticinformation and instructions for processor 1104. A storage device 1110,such as a magnetic disk or optical disk, is provided and coupled to bus1102 for storing information and instructions.

Computer system 1100 may be coupled via bus 1102 to a display 1112, suchas a cathode ray tube (CRT), for displaying information to a computeruser. An input device 1114, including alphanumeric and other keys, iscoupled to bus 1102 for communicating information and command selectionsto processor 1104. Another type of user input device is cursor control1116, such as a mouse, a trackball, or cursor direction keys forcommunicating direction information and command selections to processor1104 and for controlling cursor movement on display 1112. This inputdevice typically has two degrees of freedom in two axes, a first axis(e.g., x) and a second axis (e.g., y), that allows the device to specifypositions in a plane.

Computer system 1100 may implement the techniques described herein usingcustomized hard-wired logic, one or more ASICs or FPGAs, firmware and/orprogram logic which in combination with the computer system causes orprograms computer system 1100 to be a special-purpose machine. Accordingto one embodiment, the techniques herein are performed by computersystem 1100 in response to processor 1104 executing one or moresequences of one or more instructions contained in main memory 1106.Such instructions may be read into main memory 1106 from another storagemedium, such as storage device 1110. Execution of the sequences ofinstructions contained in main memory 1106 causes processor 1104 toperform the process steps described herein. In alternative embodiments,hard-wired circuitry may be used in place of or in combination withsoftware instructions.

The term “storage media” as used herein refers to any non-transitorymedia that store data and/or instructions that cause a machine tooperate in a specific fashion. Such storage media may comprisenon-volatile media and/or volatile media. Non-volatile media includes,for example, optical or magnetic disks, such as storage device 1110.Volatile media includes dynamic memory, such as main memory 1106. Commonforms of storage media include, for example, a floppy disk, a flexibledisk, hard disk, solid state drive, magnetic tape, or any other magneticdata storage medium, a CD-ROM, any other optical data storage medium,any physical medium with patterns of holes, a RAM, a PROM, and EPROM, aFLASH-EPROM, NVRAM, any other memory chip or cartridge.

Storage media is distinct from but may be used in conjunction withtransmission media. Transmission media participates in transferringinformation between storage media. For example, transmission mediaincludes coaxial cables, copper wire and fiber optics, including thewires that comprise bus 1102. Transmission media can also take the formof acoustic or light waves, such as those generated during radio-waveand infra-red data communications.

Various forms of media may be involved in carrying one or more sequencesof one or more instructions to processor 1104 for execution. Forexample, the instructions may initially be carried on a magnetic disk orsolid state drive of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 1100 canreceive the data on the telephone line and use an infra-red transmitterto convert the data to an infra-red signal. An infra-red detector canreceive the data carried in the infra-red signal and appropriatecircuitry can place the data on bus 1102. Bus 1102 carries the data tomain memory 1106, from which processor 1104 retrieves and executes theinstructions. The instructions received by main memory 1106 mayoptionally be stored on storage device 1110 either before or afterexecution by processor 1104.

Computer system 1100 also includes a communication interface 1118coupled to bus 1102. Communication interface 1118 provides a two-waydata communication coupling to a network link 1120 that is connected toa local network 1122. For example, communication interface 1118 may bean integrated services digital network (ISDN) card, cable modem,satellite modem, or a modem to provide a data communication connectionto a corresponding type of telephone line. As another example,communication interface 1118 may be a local area network (LAN) card toprovide a data communication connection to a compatible LAN. Wirelesslinks may also be implemented. In any such implementation, communicationinterface 1118 sends and receives electrical, electromagnetic or opticalsignals that carry digital data streams representing various types ofinformation.

Network link 1120 typically provides data communication through one ormore networks to other data devices. For example, network link 1120 mayprovide a connection through local network 1122 to a host computer 1124or to data equipment operated by an Internet Service Provider (ISP)1126. ISP 1126 in turn provides data communication services through theworld wide packet data communication network now commonly referred to asthe “Internet” 1128. Local network 1122 and Internet 1128 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 1120 and through communication interface 1118, which carrythe digital data to and from computer system 1100, are example forms oftransmission media.

Computer system 1100 can send messages and receive data, includingprogram code, through the network(s), network link 1120 andcommunication interface 1118. In the Internet example, a server 1130might transmit a requested code for an application program throughInternet 1128, ISP 1126, local network 1122 and communication interface1118.

The received code may be executed by processor 1104 as it is received,and/or stored in storage device 1110, or other non-volatile storage forlater execution.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense. The sole and exclusive indicator of the scope of the invention,and what is intended by the applicants to be the scope of the invention,is the literal and equivalent scope of the set of claims that issue fromthis application, in the specific form in which such claims issue,including any subsequent correction.

What is claimed is:
 1. One or more non-transitory computer readablemedia storing instructions which, when executed by one or more hardwareprocessors, cause: identifying a request to store a first value, of afirst type, as a value of a first field that (a) corresponds to aparticular object and (b) is referenced by a particular field name;determining that the particular object is associated with a particulartype; determining that the particular type comprises: (a) a second fieldthat (i) is referenced by the same particular field name indicated bythe request and (ii) is of the first type; (b) a third field that (i) isreferenced by the same particular field name indicated by the requestand (ii) is of a second type different from the first type; executing aconversion function to convert the first value, that is indicated by therequest, into a second value of the second type; storing the secondvalue of the second type into the third field that (a) corresponds tothe particular object and (b) is referenced by the particular fieldname.
 2. The one or more media of claim 1, wherein the second field andthe third field are different versions of a migrated field.
 3. The oneor more media of claim 1, wherein the request further comprises storinga third value, of a third type, as another value of a fourth field that(a) corresponds to the particular object and (b) is referenced byanother field name, and further storing instructions which cause:storing the third value of the third type as the another value of thefourth field that (a) corresponds to the particular object and (b) isreferenced by the another field name.
 4. The one or more media of claim1, wherein the request is identified in a class file.
 5. The one or moremedia of claim 1, wherein: the request includes a symbolic reference tothe first field; and storing the second value of the second type intothe third field that (a) corresponds to the particular object and (b) isreferenced by the particular field name comprises: resolving thesymbolic reference as a direct reference to the third field that (a)corresponds to the particular object and (b) is referenced by theparticular field name; based on the direct reference, identifying amemory address of the third field that (a) corresponds to the particularobject and (b) is referenced by the particular field name; storing thesecond value of the second type at the memory address.
 6. The one ormore media of claim 1, wherein executing the conversion function isresponsive to: determining that the second field was migrated to thethird field.
 7. The one or more media of claim 1, wherein executing theconversion function is responsive to: determining that the first typeindicated by the request and the first type associated with the secondfield are same.
 8. The one or more media of claim 1, wherein executingthe conversion function is responsive to: determining that the firsttype indicated by the request and the second type associated with thethird field are different.
 9. The one or more media of claim 1, whereindetermining that the particular comprises (a) the second field and (b)the third field comprises: determining that a declaration or definitionof the particular type includes a first declaration of the second fieldand a second declaration of the third field.
 10. The one or more mediaof claim 1, further storing instructions which cause: determining thatthe particular type further comprises a reference to the conversionfunction.
 11. The one or more media of claim 1, further storinginstructions which cause: determining that a declaration or definitionof the particular type includes a reference to the conversion function.12. A method, comprising: identifying a request to store a first value,of a first type, as a value of a first field that (a) corresponds to aparticular object and (b) is referenced by a particular field name;determining that the particular object is associated with a particulartype; determining that the particular type comprises: (a) a second fieldthat (i) is referenced by the same particular field name indicated bythe request and (ii) is of the first type; (b) a third field that (i) isreferenced by the same particular field name indicated by the requestand (ii) is of a second type different from the first type; executing aconversion function to convert the first value, that is indicated by therequest, into a second value of the second type; storing the secondvalue of the second type into the third field that (a) corresponds tothe particular object and (b) is referenced by the particular fieldname; wherein the method is performed by one or more devices, eachincluding one or more hardware processors.
 13. The method of claim 12,wherein the second field and the third field are different versions of amigrated field.
 14. The method of claim 12, wherein: the requestincludes a symbolic reference to the first field; and storing the secondvalue of the second type into the third field that (a) corresponds tothe particular object and (b) is referenced by the particular field namecomprises: resolving the symbolic reference as a direct reference to thethird field that (a) corresponds to the particular object and (b) isreferenced by the particular field name; based on the direct reference,identifying a memory address of the third field that (a) corresponds tothe particular object and (b) is referenced by the particular fieldname; storing the second value of the second type at the memory address.15. One or more non-transitory computer readable media storinginstructions which, when executed by one or more hardware processors,cause: identifying a request to store a first value, of a first type, asa value of a first field that (a) corresponds to a particular object and(b) is referenced by a particular field name; determining that a priorversion of a migrated field, that is referenced by the same particularfield name indicated by the request, is of a second type; determiningthat a current version of the migrated field, that is referenced by thesame particular field name indicated by the request, is of a third type;responsive to determining that (a) the second type and the first typeare same and (b) the third type and the first type are different:executing a conversion function to convert the first value, that isindicated by the request, into a second value of the third type; storingthe second value of the third type into the current version of themigrated field that (a) corresponds to the particular object and (b) isreferenced by the particular field name.
 16. The one or more media ofclaim 15, further storing instructions comprising: determining that theparticular object is associated with a particular type; determining thatthe particular type includes the migrated field referenced by theparticular field name.
 17. The one or more media of claim 15, whereindetermining that the prior version of the first field is of the secondtype and determining that the current version of the first field is ofthe third type comprises: determining that a first declaration of theprior version of the migrated field indicates that the prior version ofthe migrated field is associated with the second type; and determiningthat a second declaration of the current version of the migrated fieldindicates the current version of the migrated field is associated withthe third type.
 18. The one or more media of claim 15, wherein: therequest includes a symbolic reference to the first field; and storingthe second value of the third type into the current version of themigrated field that (a) corresponds to the particular object and (b) isreferenced by the particular field name comprises: resolving thesymbolic reference as a direct reference to the current version of themigrated field that (a) corresponds to the particular object and (b) isreferenced by the particular field name; based on the direct reference,identifying a memory address of the current version of the migratedfield that (a) corresponds to the particular object and (b) isreferenced by the particular field name; storing the second value of thethird type at the memory address.
 19. The one or more media of claim 15,further storing instructions comprising: determining whether the firsttype is same as a type associated with any version of the migrated fieldof the particular type.
 20. The one or more media of claim 15, furtherstoring instructions comprising: determining whether the first type issame as the third type of the current version of the migrated field ofthe particular type.